Field of the Invention
The present invention relates to a solid-state image sensor circuit for outputting an image signal from a solid-state image sensor as a digital value.
Solid-state image sensors such as a CCD (Charge Coupled Device) are small in size and light in weight. Therefore, they are widely used in various fields, e.g., for focusing detection in a camera, image reading in a facsimile system or copy machine, and for a sensor of an assembly robot.
An image signal output from a solid-state image sensor is normally converted into a digital signal to be processed. However, the output from the image sensor may include an output component which is generated due to a dark current regardless of light reception, and hence accurate image data cannot be obtained unless such a component is removed from the image signal.
Therefore, some of light-receiving elements of a conventional image sensor are covered with an aluminum film to form a shielded portion and an output (a dark output) therefrom is subtracted from the sensor output by a differential amplifier, thereby removing the dark output (Japanese Patent Application No. 60-108815). In another conventional image sensor, since dark outputs vary depending on light-receiving elements, a shutter is provided to each light-receiving element. A dark output from each element, which is obtained while light is shielded by a corresponding shutter, is stored in a memory, and each stored dark output is subtracted from an image signal (Japanese Patent Application No. 60-202424). In still another conventional image sensor, a dark output is calculated when an apparatus is assembled and stored in a nonvolatile memory. When an image signal is calculated, the dark output is read out from the memory, and subtracted from the dark output component (Japanese Patent Application No. 57-16173).
However, in such conventional circuits, a subtraction circuit, a shutter mechanism, or a memory is required, resulting in a complicated circuit arrangement. In addition, if a dark output component stored during assembly of the apparatus is subtracted, changes in dark output due to changes in temperature after assembly cannot be obtained, and hence an accurate image signal cannot be obtained.